A “central listener” is one located within an ideal listening area (or “sweet spot”), for example, equidistantly with respect to a pair of stereo loudspeakers. An “off-center” listener is one located outside such an ideal listening area. In a two loudspeaker stereo arrangement, a central listener perceives “phantom” or “virtual” sound images generally at their intended locations between the loudspeakers, whereas an off-center listener perceives such virtual sound images as closer to the loudspeaker with respect to which the listener is nearer. This effect increases as the listener becomes more and more off-center (i.e., the virtual sound images become closer and closer to the nearer loudspeaker).
It is known to take two-channel, left and right, stereo audio signals, and from them derive a central loudspeaker feed derived from a combination of the original signals. In some known systems the combination is variable. Some known systems also vary the gain to the left and right loudspeaker feeds as well. The gains in the various paths typically are controlled by analysis of the directional information contained in the stereo input signals. See, for example, U.S. Pat. No. 4,024,344. The purpose of such center-channel derivations is to counteract the above-mentioned effect for off-center listeners such that sound images, particularly central sound images, are perceived as coming from their intended locations. Unfortunately, an unwanted side-effect of employing such a derived center channel is the degradation (narrowing) of the stereo image for central listeners—sound imaging improvements for off-center listeners cause sound imaging deterioration for central listeners. A central listener does not need a center channel loudspeaker in order to perceive sound images at their intended locations. Thus, there is a need to balance the soundfield improvement for some listeners against the soundfield degradation for others.